1. Field of the Invention
This invention relates to an image forming device which utilizes a continuous web of image transfer material such as fanfold paper, rolled paper, and the like. More particularly, the invention is concerned with an image transfer device in such image forming apparatus, by which an image such as an electrostatic or magnetic latent image, toner image, and so forth formed on a recording element in the image forming apparatus such as a photosensitive drum, an insulative/electrically conductive drum, etc. is transferred onto an image transfer material such as paper, etc.
2. Description of the Prior Art
As the image forming apparatus using a continuous web of image transfer material, there have so far been put into practice various kinds of recording devices, in which the electrophotographic method and the magnetization method are applied. For instance, there is a recording device which performs the recording operations by irradiating a modulated laser beam onto an ordinary electrophotosensitive material. This type of recording device first modulates light quantity of the laser beam by an output data from a computer, etc., then focuses this modulated laser beam onto the surface of a drum-shaped photosensitive member (hereinafter simply called "drum") as an information recording element by means of a scanning means such as a rotatory mirror, etc., and scans the focused image for exposure. The drum, through various electrophotographic processes, is subjected to the light exposure by the abovementioned laser beam, and forms thereon an electrostatic potential image. This electrostatic image is then developed by toner to render it visible, after which the visible image is transferred onto an image transfer material, followed by further heating or pressure application to fix the image or the image transfer material, thereby attaining the image recording.
The image recording device of the abovementioned type is capable of high speed printing on account of its absence of a mechanical printing section as in conventional line printers, hence it provides an improved non-impact type printer of high image quality and low noise.
Generally, the image transfer device which transfers a toner image onto an image transfer material is constructed with a corona discharger having excellent image transfer efficiency, and a guide plate which guides a predetermined image transfer material into a predetermined feeding path. The corona discharger is disposed in confrontation to the drum at the center of the image transfer section, while the guide plate is disposed at a position where the image transfer material becomes able to contact the drum on its tangential line at the center of the image transfer section. By always maintaining constant the distance between the surface of the drum and the surface of the image transfer material on the guide plate, the image transfer material is able to travel stably along the guide member, whereby a transferred (or reproduced) image of high transfer efficiency and high image quality can be obtained. If the distance between them becomes inappropriate, the state of contact between the image transfer material and the photosensitive drum lacks stability with the consequence that not only the image transfer efficiency and the image quality lower, but also, when the drum and the guide plate get into strong frictional contact through the image transfer material, they will become mutually impaired, or the image transfer material is damaged to interrupt the recording operation. Accordingly, it is indispensable that the distance between the drum surface and the surface of the image transfer material be maintained constant all the time.
Incidentally, it is usual that the guide plate to guide the image transfer material has some errors in its mechanical precision due to cylindrically and rectilinearity derived from the manufacturing errors, or owing to non-uniformity in working during the manufacturing steps. It has been contemplated to minimize such defects in the manufacture by increasing the working precision, which however accompanies real difficulties due to requirement for high degree of expertise in the working technique and prohibitive increase in the manufacturing cost. Therefore, demands has been continuing for a mechanism to supplement the defects in the mechanical precision of the guide plate without necessity for high degree of working technique and prohibitive increase in the manufacturing cost.
By the way, thickness of the image transfer material in the form of a continuous web of paper to be used for such image forming apparatus is standardized by a weight indication usually called "basis weight". For example, there are six kinds of paper thickness for the image transfer material to be used for general recording devices, which ranges from the thinnest one of 45 kg in basis weight to the thickest one of 135 kg in basis weight. Converting these basis bulk weight measurements to their respective paper thickness in metric system, they range from 0.068 mm to 0.190 mm. It is therefore necessary that these kinds of paper be conveyed in a predetermined state of contact with the drum so that a reproduced image of good quality may always be obtained, when they are used as the image transfer material. In more detail, in order to enable the image transfer material having at least six kinds of thickness to contact the drum on the tangential line thereof, there becomes necessary to provide a mechanism capable of varying the distance between the surface of the drum and the surface of the image transfer material to be guided by the guide plate in accordance with thickness of the image transfer material to be used.
Further, in the non-impact printer of the abovementioned construction, it is necessary that the image transfer material guided by the guide plate be in contact with the drum surface at the time of the image transfer during a period of from start of the recording operations by the device until its stoppage, i.e., while the image transfer material is being conveyed to the drum, and that it be separated from the drum surface outside the image transfer operation, i.e., when the image transfer material is not conveyed. If such operation is to be done by the guide plate, the plate is required, as its condition for forming reproduced image of high quality, to have sufficient response to the operating surface running stably and an extremely high speed.